This invention relates generally to devices for insufflating gas into a mass of molten metal, e.g., steel, and methods of making such devices.
The making of steel or other metals typically involves the introduction of gases into the ladle or vessel holding the molten metal in order to stir it. The gas is typically introduced into the ladle via a device called a stirring plug. Such a plug may be mounted in the bottom or side of the vessel. Prior art stirring plugs have taken numerous forms and constructions.
For example, one common type of stirring plug comprises a solid, non-gas-permeable, conical refractory member disposed within a loose fitting metal or ceramic shell or canister. Such a "canistered" plug is commonly disposed within a seating block in the wall, e.g., the bottom, of the vessel holding the molten metal, and the purging gas is transported through the gap between the refractory cone and the metal canister into the molten metal in the vessel.
In U.S. patent application Ser. No. 07/362,998, filed on June 8, 1989, entitled Refractory Gas Stir Plugs With Interconnecting Surface Channels And Method Of Making The Same, and in U.S. patent application Ser. No. 07/363,240, filed on June 8, 1989, entitled Gas Stir Plugs With Multichannel Monograin Surface And Method Of Making The Same, there are disclosed and claimed "canistered" plugs which overcome the disadvantages of various prior art canistered stirring plugs having gas passageways formed between the refractory cone and the metal canister.
Another common type of stirring plug comprises a conical shaped member or plug formed of a porous refractory material through which the purging (stirring) gas is passed to produce fine gas bubbles to stir the molten metal. Thus, that type of plug utilizes the porosity of the material forming the plug to create a capillary system formed by the interstitial spaces between the porous material for carrying the stirring gas through the plug. Such plugs are commonly disposed within a seating block in the wall, e.g., bottom, of the vessel.
Another type of uncanistered plug is the so-called directed porosity plug. That plug comprises a conical body of cast refractory material containing an array of fine, e.g., 0.7 mm diameter, channels that run in a straight line from the bottom to the top of the plug. When these plugs are used the gas is distributed very finely in the melt by means of the capillaries, but as it passes through the capillaries it undergoes a very high degree of friction loss as a result of the turbulence which develops on the inside surfaces of the capillaries. The effects of this turbulence on the flow of gas decreases with increasing size of the capillary cross section. Thus, it is not possible to increase the diameter of the capillaries to any desired extent in order to minimize friction since such action would enable the molten metal to penetrate too deeply into the capillaries and block them in the event that the flow of gas should cease.
Only a large number of capillaries can guarantee the very high gas flow rate frequently desired in a steel mill. From the production angle, however, this turns out to be very expensive. Thus, to reduce friction losses, it was found advantageous to form a conical stirring plug of a single or multi-part construction to provide plural identical joints (in the case of a multi-part construction) or slots or passageways (in the case of a single part construction) extending linearly from the bottom to the top of the plug. Such "jointed/slotted" plugs exhibit similar gas agitation properties as the capillary tube plugs, but with significantly smaller pressure losses.
The joints or passageways in one type of conical "jointed/slotted" plug, such as shown in German Patent No. DE 3,538,498, are generally of arcuate shape in transverse cross-section, i.e., perpendicular to the longitudinal central axis of the conical plug, with the diameter of the arc sections decreasing linearly from the bottom of the plug to its top. Moreover, the width of each passageway at the top of the plug (such width being typically within the range of 10-30 mm) is less than at the bottom of the plug (such width being typically within the range of 25-50 mm), with the thickness of the passageway being constant along its length (such thickness being typically within the range of 0.1-1 mm). Such conicity of passageways serves to minimize the friction loss as the purging or stirring gas passes through the passageways. Moreover, this type of plug exhibits a high gas flow rate per unit of time (such as that achieved by use of a very large number of capillaries) with a small number of passageways. However, the conically upward-tapering geometry of the passageways leads to very low friction losses, the gas-conducting open space is difficult to blow out or blast free of metal should the molten metal run into them and solidify, because the solidified metal forms a wedge and thus locks itself in place, preventing its ejection. Moreover, if the plug is formed of a multi-part design, e.g., an internal mandrel located within an external sleeve, with the joints being located at the interface of the parts, the parts can shift location with respect to each other and thus increase the thickness of the joint. The immediate consequence of such action would be agitation failure, because molten metal could run too far into the plug and thereby block the gas-conducting free space.
In another type of conical "jointed/slotted" plug, such as shown in German Patent No. DE 3,625,117, each of the passageways is rectangular and of constant cross-section from the bottom of the plug to its top. The passageways are arranged in a radial, starburst configuration, with their longitudinal central axes being located in a cylindrical locus. This type of plug also exhibits a high gas flow rate per unit of time by use of a relatively small number of passageways. However, the radial, starburst design of the passageways leads to an undefined cracking of the passageways toward the outside edge of the plug. Such action occurs because, in the case of a steel-blocked passageway, the gas pressure being exerted on the remaining webs, i.e., the material between the outside edge of the passageway and the outer surface of the plug, creates considerable tensile stress.
Disposing the passageways in a cylindrical array within the plug prevents cracking toward the outer surface of the plug, but is not without its own disadvantage. In this regard if the plug is constructed so that the passageways are disposed in a cylindrical array, with their longitudinal central axes being located in a cylindrical locus, the distances separating the immediately adjacent passageways from each other is constant over the entire height of the plug. The pressures and temperature changes occurring during operation lead to cracking, primarily from the edge of one passageway to the edge of an immediately adjacent passageway. This can lead to the formation of circular cracks within the body of the plug. Such cracks may extend the entire height of the plug, i.e., become cylindrical. Moreover, because the spacing between the immediately adjacent passageways in a cylindrically disposed arrangement is of equal width from the bottom of the plug to the top, such circular cracks can readily propagate over the entire height of the plug and lead to the breaking off of a piece of the plug at any point.
In order to reduce the pressure loss problem inherent in plugs having joints or passageways therein through which the stirring gas passes the joints/passageways are sometimes constructed to increase in cross sectional are from the top to the bottom of the plug. High volume gas flow through such plugs is achieved through use of a small number of joints/passageways (as opposed to a large number of capillaries as is the case with porous or capillaried plugs). Plugs utilizing increasing cross sectional area joints/passageways, while producing low friction losses, nevertheless have the disadvantage that as the plug wears down during use molten metal penetration increases after the gas flow is terminated due to the increased cross sectional area of the joints/passageways. Further still such plugs (due to the decreased cross sectional area of the joints/passageways going from the bottom of the plug to the top) have the disadvantage of increasing the difficulty in blowing out penetrated molten metal from the joints/passageways.
Prior art stirring plugs are also found in the following U.S. Pat. Nos. 4,535,975 (Buhrmann et al.), 4,539,043 (Miyawaki et al.), 4,647,020 (Leisch), 4,741,515 (Sharma et al.), 4,836,433 (Perry), 4,840,356 (Labate), 4,858,894 (Labate), 4,884,787 (Dotsch et al.), 4,898,369 (Perry), 4,905,971 (Rothfuss et al.), and 4,925,166 (Zimmermann), and in European Patents Applications: EP 311,785 A1, and EP 326,882 A2.